1) Field of the Invention
The present invention relates to a flexible flat cable (FFC) that is used to connect between printed circuit boards.
2) Description of the Related Art
FFCs are used to connect between printed circuit boards of various kinds of electronic devices. One-side strip conductor type FFCs are known in the art. In the one-side strip conductor type FFC, a plurality of conductors is laid out at specific distance and an insulator is placed between the adjacent conductors thereby forming a conductor unit. Subsequently, the front and the back surfaces of the conductor unit are laminated with an insulation sheet, then, the conductor unit is stripped at both ends, and a reinforcing tape is bonded to one of the surfaces.
To have a better connection performance, a metal plating is provided on the surface of the conductors. Leaded solder was used as the plating material in the past. However, because lead is toxic, non-leaded tin alloy plating is becoming popular day-by-day. However, there is a problem with the tin alloy plating that whisker is generated.
Whisker is a whiskered crystalline product that is generated on the surface of a plated surface. The generation of the whisker is not desirable; because, the whisker can short-circuit the conductors. The whisker can be generated due to various reasons. For example, the whisker can be generated due to mechanical residual stress in the plated conductor, or due to chemical corrosion of the plated surface due to reaction with metal particles and/or humidity.
FIG. 1 is a flowchart of a method of manufacturing the conventional FFCs. At step S1, a conductor material is rolled, a tin alloy is plated onto the rolled conductor material. At step S2, an electric current is passed through the conductor material to heat treat the conductor material. This heat treatment suppress the generation of whisker. Due to the heat treatment, the tin-alloy plating is re-crystallized. At step S3, plural slits are formed on the conductor material, thereby forming a plurality of conductors laid out with a predetermined interval therebetween. At step S4, an insulation tape is bonded to the conductors in such a manner that there is the insulation tape between adjacent conductors, and a reinforcing tape is bonded to one of both end surfaces, thereby forming a conductor unit including the conductors and the insulation tape. At step S5, an insulation sheet is thermally bonded to the front and the back surfaces of the conductor unit, thereby laminating the conductor unit with the insulation sheet. At step S6, the FFC is packed and shipped.
Japanese Patent Application Laid-Open No. 2001-43743 discloses a different method. A conductor is configured by a tin-copper alloy plated pillar conductor that is provided with a tin-copper alloy plated layer including copper by 0.1 weight percent to 5.0 weight percent and tin and an unavoidable impurity by the rest of the weight on the external periphery of a conductor strand. This conductor suppresses the generation of whisker, and improves solder wetness.
In the method explained with reference to FIG. 1, the conductor processing (step S3) and the bonding of insulation tape (step S4) are performed after performing the heat treatment (step S2). The conductor processing (step S3) and the bonding of insulation tape (step S4) include processes such as slit formation, cutting, and pressing that exert external stress on the conductor unit. This external stress generates the whisker.
In the method disclosed in Japanese Patent Application Laid-Open No. 2001-43743, plating of the conductor material is complex and troublesome process. Moreover, this method also requires that the conductor processing (step S2) and the bonding of insulation tape (step S4) be carried out. Because the conductor processing (step S2) and the bonding of insulation tape (step S4) exert stress on the conductor unit, the whisker is generated anyway.